1. Field of the Invention
The present invention relates to a full time four wheel drive system for use in a vehicle and more particularly, to an improved four wheel drive system including a housing in the vicinity of a transmission and an engine, a central differential gear assembly, a viscous coupling assembly, and a front wheel differential gear assembly disposed in a parallel relationship within the housing for effectively operating the four wheel drive system.
2. Description of the Prior Art
Various types of full time four wheel drive systems in front wheel drive vehicles are well known in the art. Such four wheel drive systems include a front wheel differential gear, a rear wheel differential gear, and a center differential gear and a viscous coupling for distributing power to both front wheel and rear wheel differential gears. Such a viscous coupling contains a cylindrical container having a pair of discs with a plurality of apertures disposed on the discs. A plurality of pairs of discs are alternatively stacked with respect to each other wherein one disc is connected to one side shaft and the other disc is connected to another side shaft. High viscosity silicon oil is filled into the cylindrical container and the container is sealed. The silicon oil fills the spaces between the pairs of discs and functions to transfer power. Thus the silicon oil provides connection to both shafts in a uniform motion or in a differential motion. Also, such four wheel drive systems are structurally classified by (1) the center differential gear and the viscous coupling disposed in the front wheel differential gear or (2) the viscous coupling disposed between the rear axle shafts in order to eliminate the rear wheel differential gear.